1. Field of the Invention
The present invention relates, generally, to organic semiconductor devices, and, in particular embodiments, to processes for fabricating organic semiconductor devices using ink-jet printing technology and devices and systems employing the same.
2. Description of Related Art
Inorganic semiconductors, such as silicon, are often used to produce modern semiconducting and photonic devices. The processing of these inorganic semiconductor devices can be complicated and costly, and typically includes process steps such as the growing of crystals, the slicing and polishing of wafers, and building of integrated electronic circuits on the wafer. By comparison, conventional polymers (sometimes referred to as plastics) are relative easy to process. For example, the fabrication of traditional plastic parts may include relatively simple process steps such as the injection of molten plastic material into molds. Conventional polymers are also flexible, lightweight, and can be fabricated over large surface areas. However, conventional plastics are not semiconducting, and are therefore unsuitable for the fabrication of semiconductor devices.
Conjugated polymer is an organic material that combines the electrical and optical properties of semiconductors and the processability of conventional plastics. The semiconducting properties of conjugated polymers originate from the delocalized pi orbitals formed in carbon containing compounds, such as poly(phenylenevinylene), polythiophene (PT), and poly(2-methoxy-5-(2xe2x80x2-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV). Unlike conventional polymers, conjugated polymers contain double bonds which make the material semiconducting rather than insulating. Conjugated polymers retain the low-cost processing benefits, flexibility, light weight, and large-scale producibility of conventional polymers and the general semiconducting characteristics of silicon.
Conjugated polymer devices are typically fabricated by spin-coating. Spin-coating takes advantage of the solution processability of polymer by spinning a substrate containing a large drop of liquid conjugated polymer at high velocity about an axis, causing the liquid conjugated polymer to flow outward and coat the substrate with a thin film of material. However, there are disadvantages associated with spin-coating. Spin-coating results in solution wastage, as the majority of the liquid conjugated polymer flies off the substrate instead of coating the surface. In addition, spin-coating is sensitive to dust or other imperfections on the surface of the substrate, for any projection will cause a shadow effect as the liquid organic material spreads across the surface of the substrate, leaving a radial trace of relatively thin organic material behind the imperfection.
The relatively uncontrolled flow of liquid conjugated polymer during spin-coating also does not allow for the formation of desired patterns, which limits the commercial applicability of conjugated polymers. For example, luminescent conjugated polymer sandwiched between two electrodes may be used for fabricating LEDs and light-emitting logos (LELs), but the single unpatterned layer of conjugated polymer produced by spin-coating limits such devices to a single color and requires that any patterning occur in the electrodes. Furthermore, photolithography techniques normally useful for creating patterned electrodes may not be used to pattern the layer of conjugated polymer, because the double bonds in the conjugated organic material will be destroyed by the photolithographic process.
Another class of organic semiconducting materials is the class of conjugated small organic molecules. Conjugated organic compounds (organics) are defined herein to include polymers (organics with more than two repeating units per molecular chain) and small organic molecules (organics comprised of single molecules). Small organic molecules share similar physical (electronic and optical) properties with conjugated polymers, but utilize somewhat different processing techniques. Organic molecules are usually processed using thermal sublimation at ultra high vacuum environments to form desired thin films, with typical thickness of about 100 nm. Organic molecules often use device structures similar to those used with conjugated polymers, i.e. organic thin films sandwiched between two electrodes. The patterning of the organic thin films can be achieved using a shadow mask, but this method requires the precise alignment of the shadow mask, and it is a slow and costly process. Furthermore, lateral resolution is also limited. Organic molecules can also be processed using the conventional spin-coating process, but this method lacks patterning capability. Organic molecules are typically processed by blending the molecules with a host conjugated polymer, such that the blend retains the advantageous mechanical properties of polymer for film forming purposes. Examples of typical organic compounds suitable for the buffer layers and for the ink-jet printing deposits are given in FIGS. 18a, 18b, and 18c. 
Ink-jet printing (IJP) technology, a popular technology for desktop publishing, may be used for depositing patterned conjugated organic material with high resolution. The application of IJP to deposit patterned conjugated organics has been demonstrated in an article entitled xe2x80x9cInk-jet printing of doped polymers for organic light emitting devicesxe2x80x9d by T. R. Hebner et al., Applied Physics Letters, Vol. 72, p. 519 (1998), incorporated herein by reference. However, a low concentration of dye-containing polymer solution was printed in order to use existing IJP technology. The result was a poor film unsuitable for high quality semiconductor devices.
Even where suitable patterns of conjugated organics can be deposited over a lower electrode, other problems exist. Because of the dot-forming nature of IJP, organic film printed using IJP may contain pin holes. The deposition of the upper electrode material over the patterned conjugated organic thin film may result in some of the upper electrode material coming into contact with the lower electrode through the pin holes, creating short that renders the device unusable.
Therefore, it is an object of embodiments of the present invention to provide a process for fabricating organic semiconductor devices using hybrid ink-jet printing technology, and systems and devices incorporating the same, that is relatively insensitive to substrate surface imperfections, and combines the electrical and optical properties of conventional semiconductors and the low-cost processability, flexibility, light weight, and large-scale producibility of conventional organics.
It is a further object of embodiments of the present invention to provide a process for fabricating organic semiconductor devices using hybrid ink-jet printing technology, and systems and devices incorporating the same, that allows the formation of precisely patterned single or multi-color emissive displays, devices, logos, and gray-scale pictures, including isolated emissive areas.
It is a further object of embodiments of the present invention to provide a process for fabricating organic semiconductor devices using hybrid ink-jet printing technology, and systems and devices incorporating the same, that allows the formation of high quality shadow masks for the fabrication of semiconductor devices, bio-sensors, photovoltaic devices, and photodetectors.
These and other objects are accomplished according to an emissive system for presenting visual images. The emissive system typically contains first electrodes deposited over and in contact with a substrate. One or more conjugated organic buffer layers are then deposited over and in contact with the first electrodes, and second electrodes are subsequently deposited over the conjugated organic buffer layers. The conjugated organic buffer layers regulate current flow between the first electrodes and the second electrodes. Either before or after the deposition of each conjugated organic buffer layer, but before the deposition of the second electrodes, conjugated organic deposits are ink-jet printed such that they are in contact with at least one conjugated organic buffer layer.
The conjugated organic deposits help to generate an indicator when a voltage stimulus is applied across the first electrodes and the second electrodes. Depending on the material of the conjugated organic deposits, the indicator may be luminescence, fluorescence, conductivity, or the like. A voltage source is used for selectively applying the voltage stimulus across the first electrodes and the second electrodes.
These and other objects, features, and advantages of embodiments of the invention will be apparent to those skilled in the art from the following detailed description of embodiments of the invention, when read with the drawings and appended claims.